Tortoise

One recent chelonian differs from all the others in the structure of its shell, the elements which normally compose this structure being almost all absent and their function being assumed by a mosaic of small polygonal plates. On this account the animal has been placed in a separate group, the ATHECAE, to contrast with the others which are united as the THECOPHORA. This creature, the Luth or Leathery Turtle (Sphargis coriacea) is a marine, turtle-like animal inhabiting tropical seas and is the largest living chelonian; it reaches a length of 8 feet and may weigh nearly a ton; the back is devoid of horny plates and is covered by soft skin, the food consists of molluscs, crustaceans and fish and the eggs, like those of the other marine turtles, are laid on land ; the female repairs to a sandy beach during the night, digs a shallow hole in which the eggs are deposited, covers them up, and returns to the sea; the chief breeding places appear to be the islands of the West Indies.

Thecophora.—These are subdivisible into 3 main groups, the Trionychoidea aquatic forms with a bony carapace and plastron but without horny plates, the Cryptodira, terrestrial and aquatic forms with a bony carapace and plastron covered with horny epi dermal shields and retracting the neck by a vertical S-shaped bend . and the Pleurodira resembling the Cryptodira but retracting the neck by a simple lateral flexure. Of these groups the Cryptodira contains the largest number and the majority of the typical tor ' toises; its various families, based largely on characters of the bony shell, may be recognised by the following key based on external characters only: I. Digits more or less distinct with 4 or 5 claws; terrestrial or fresh-water.

A. Pectoral shield (the 3rd paired shield of the plastron) separated from the marginal shields of the carapace by a series of small "infra-marginal" plates.

(r) Tail long and crested; plastron small and cruciform.

American.

Chelydridae.

(2) Tail long, covered with rings of scales; plastron large.

Asiatic. Platysternidae.

(3) Tail short. North American.

Kinosternidae.

Dermatemyaidae.

B. Pectoral shields in contact with the marginals of the cara pace. Testudinidae.

II. Limbs paddle-shaped with one or two claws; marine. Chelonidae. Chelydridae.—Alligator Terrapins or Snapping Turtles. Fresh water forms of north and central America, characterised by relatively huge heads, long alligator-like tails, rough shells and numerous soft projecting folds of skin on the neck and limbs. The two better known species, the Common Snapper (Chelydra serpentine) of the eastern United States and the Alligator Snapper (Macrochelys temoninckii) of the Mississippi and Missouri basins, are both large, powerful creatures, and notorious for their vicious nature and the power of their jaws.

Platysternidae.—A single species (Platysternon megacepha lum) found in fresh waters in S. China, Burma and Siam. The head is relatively very large and the neck long, a 14-inch specimen only measuring 5 inches along the carapace.

Kinosternidae.

These are the Musk Turtles of North and Central America, so called from the rather offensive odour of the secretion of their inguinal glands; they are small, aquatic crea tures, rarely exceeding a total length of one foot and in many the front and hind lobes of the plastron are hinged and can be drawn up to the carapace when the head and legs are withdrawn into the shell. Sternotherus odoratus the Stink Pot Terrapin of eastern North America is the best known species.

Dermatemydidae.—Small Central American terrapins closely allied to and resembling the members of the preceding family.

Testudinidae.

This family, containing the majority of tor toises is world wide in its distribution except the Australian region and its members show all gradations between thoroughly aquatic and exclusively terrestrial species. The Batagur (Batagur baska) of Bengal, Burma and the Malayan region is exclusively aquatic ; it attains a length (carapace only) of nearly 2 feet, is rather flattened and has the digits fully webbed. Chrysemys of eastern N. America is also exclusively aquatic and the species of this and allied genera are extensively eaten ; they are sold as "Slider Ter rapins" to distinguish them from the Diamond Back Terrapins (Malaclemys) beloved by epicures.

Clemmys, with species in S. Europe and N. Africa (C. leprosp), Asia and N. America (e.g., C. inscu/pta the Wood Terrapin), is less aquatic than the preceding and the digital webs are cor respondingly reduced. Still less aquatic are the species of Emys, E. orbicularis of Central Europe and Western Asia and E. blandingi of southern Canada and the north-eastern United States, which have indications of a transverse hinge across the plastron. This plastral hinge reaches a high degree of develop ment in the North American Box Turtles (Terrapene) which can withdraw completely into their shell and close both openings; in structure these animals are water-tortoises, but they are terrestrial in habits and have the high, domed shell of the exclusively ter restrial genera such as Kinixys, Pyxis and Testudo. Testudo is an assemblage of herbivorous, terrestrial forms and contains, among others, the well-known Giant Tortoises, some of which reach a shell length of over 4 feet and may weigh over 600 lb. There were three groups of these animals, one confined to the islands of the Galapagos Group where every small island had its own race and the largest several, another to the Mascarene Islands (Bourbon, Mauritius Rodriguez) and the third to the Seychelles-Aldabra Group. Before the advent of man these crea tures abounded in almost incredible numbers, but it was soon dis covered that, on account of their ability to live for months without food, they formed an excellent supply of easily transported fresh meat for sailing vessels and their numbers were rapidly decimated.